Optical microphone

Abstract

There is provided an optical microphone for detecting an acoustic wave propagating in an ambient fluid, the optical microphone including: a propagation medium section; a light source for emitting a light wave to be transmitted through a diffraction region in the propagation medium section; and a photoelectric conversion section for detecting the light wave having been transmitted through the propagation medium section. A first acoustic wave which is a portion of the acoustic wave and a second acoustic wave which is another portion thereof are allowed to propagate in the propagation medium section so as to simultaneously arrive at the diffraction region, and an interference component between a +1 st order diffracted light wave and a −1 st order diffracted light wave of the light wave generated based on a refractive index distribution of the propagation medium occurring in the diffraction region.

What is claimed is: 
     
       1. An optical microphone for detecting an acoustic wave by using a light wave, the acoustic wave propagating in an ambient fluid, comprising:
 a propagation medium section for the acoustic wave to propagate through; 
 a light source for emitting a light wave to be transmitted through a diffraction region in the propagation medium section; and 
 a photoelectric conversion section for detecting the light wave having been transmitted through the propagation medium section and outputting an electrical signal, wherein, 
 the optical microphone is configured: 
 to allow a first acoustic wave and a second acoustic wave to propagate in antiparallel directions in the propagation medium section so as to simultaneously arrive at the diffraction region and traverse the light wave being transmitted through the diffraction region, the first acoustic wave being a portion of the acoustic wave and the second acoustic wave being at least a portion of the remainder; and 
 to generate, in the diffraction region, a +1 st  order diffracted light wave and a −1 st  order diffracted light wave of the light wave based on a refractive index distribution of a propagation medium composing the propagation medium section, the refractive index distribution occurring due to propagation of the first acoustic wave and the second acoustic wave; and 
 the photoelectric conversion section detects at least one of: an interference component between a +1 st  order diffracted light wave of the light wave ascribable to the first acoustic wave and a −1 st  order diffracted light wave of the light wave ascribable to the second acoustic wave; and 
 another interference component between a −1 st  order diffracted light wave of the light wave ascribable to the first acoustic wave and a +1 st  order diffracted light wave of the light wave ascribable to the second acoustic wave. 
 
     
     
       2. The optical microphone of  claim 1 , wherein the propagation medium section includes first and second input aperture planes opposite from each other, the first acoustic wave and the second acoustic wave being respectively incident to the first and second input aperture planes. 
     
     
       3. The optical microphone of  claim 2 , wherein the first and second input aperture planes of the propagation medium section are positioned equidistant from the diffraction region. 
     
     
       4. The optical microphone of  claim 3 , further comprising a waveguide structure having:
 first and second input apertures facing in a same direction; 
 first and second output apertures opposing each other; and 
 first and second waveguides provided respectively between the first and second input apertures and the first and second output apertures, the waveguide structure guiding the first acoustic wave entering at the first input aperture and the second acoustic wave entering at the second input aperture respectively to the first and second output apertures, wherein 
 the first and second output apertures of the waveguide structure are disposed on the first and second input aperture planes of the propagation medium section, respectively. 
 
     
     
       5. The optical microphone of  claim 4 , wherein the first and second waveguides are disposed symmetrically in the waveguide structure. 
     
     
       6. The optical microphone of  claim 4 , further comprising a horn connected to the first and second input apertures of the waveguide structure. 
     
     
       7. The optical microphone of  claim 1 , wherein the first acoustic wave and the second acoustic wave are transmitted through a same terrain in the diffraction region. 
     
     
       8. The optical microphone of  claim 1 , wherein the first acoustic wave and the second acoustic wave are transmitted through different terrains in the diffraction region. 
     
     
       9. The optical microphone of  claim 8 , wherein,
 the propagation medium section includes a first propagation medium portion and a second propagation medium portion; 
 the diffraction region includes a first diffraction subregion and a second diffraction subregion respectively positioned in the first propagation medium portion and the second propagation medium portion; and 
 between the light source and the photoelectric conversion section, the first diffraction subregion and the second diffraction subregion are together disposed so that one is overlaid on the other. 
 
     
     
       10. The optical microphone of  claim 1 , wherein,
 the photoelectric conversion section is disposed so as to be shifted, along a direction along which the first acoustic wave and the second acoustic wave propagate, from the light wave having been transmitted through the diffraction region, and 
 detects only one of: the interference component between the +1 st  order diffracted light wave of the light wave ascribable to the first acoustic wave and the −1 st  order diffracted light wave of the light wave ascribable to the second acoustic wave; and the other interference component between the −1 st  order diffracted light wave of the light wave ascribable to the first acoustic wave and the +1 st  order diffracted light wave of the light wave ascribable to the second acoustic wave. 
 
     
     
       11. The optical microphone of  claim 1 , further comprising, between the photoelectric conversion section and the diffraction region in the propagation medium section, a blocking section for blocking the light wave having been transmitted through the diffraction region so that a part or a whole of the light wave having been transmitted through the diffraction region is prevented from entering the photoelectric conversion section. 
     
     
       12. The optical microphone of  claim 1 , further comprising, between the propagation medium and the photoelectric conversion section, an optical element for changing directions of propagation of the +1 st  order diffracted light wave and the −1 st  order diffracted light wave of the light wave. 
     
     
       13. The optical microphone of  claim 1 , wherein the propagation medium section has an acoustic velocity smaller than that of air, and is composed of a propagation medium in solid form. 
     
     
       14. The optical microphone of  claim 13 , wherein the propagation medium is composed of a dry silica gel. 
     
     
       15. The optical microphone of  claim 1 , further comprising a frequency conversion section for converting a frequency of the electrical signal obtained at the photoelectric conversion section into ½. 
     
     
       16. A method of detecting an acoustic wave propagating in an ambient fluid by using a light wave, the method comprising:
 a step of allowing a first acoustic wave and a second acoustic wave to propagate in antiparallel directions in the propagation medium section so as to simultaneously arrive at a diffraction region of the propagation medium section, the first acoustic wave being a portion of the acoustic wave and the second acoustic wave being at least a portion of the remainder; 
 a step of allowing a light wave to be transmitted through the diffraction region of the propagation medium section so as to traverse the propagating first acoustic wave and second acoustic wave, and generating a +1 st  order diffracted light wave and a −1 st  order diffracted light wave of the light wave in the diffraction region based on a refractive index distribution of a propagation medium composing the propagation medium section, the refractive index distribution occurring due to propagation of the first acoustic wave and the second acoustic wave; and 
 a step of detecting at least one of: an interference component between a +1 st  order diffracted light wave of the light wave ascribable to the first acoustic wave and a −1 st  order diffracted light wave of the light wave ascribable to the second acoustic wave; and another interference component between a −1 st  order diffracted light wave of the light wave ascribable to the first acoustic wave and a +1 st  order diffracted light wave of the light wave ascribable to the second acoustic wave.